Lime, or quicklime, is the oxide of calcium, CaO, and is commonly obtained by calcining limestone. Limestone is calcined in two main types of kilns, vertical or shaft kilns, and horizontal, rotary kilns.
Shaft kilns are of two main varieties, single shaft and multiple shaft. In both, solid particulate matter (limestone or other mineral aggregate) is loaded into the kiln shaft or shafts from the the top of the kiln and slowly travel down the shaft. In a single shaft kiln, the flow of gas is counter-current to the travel of limestone. In a multiple shaft or so-called "regenerative" kiln a crossover duct is provided between lower portions of the shafts and not all of the shafts are active at the same time. Air travels downwardly through the active shaft and crosses to the other shaft and flows upwardly therethrough for preheating of the aggregate prior to activation of the shaft.
For example, in a double shaft kiln, only one shaft is active at a time. During the active phase fuel, such as powdered coal, is introduced into the shaft via lances and combustion gases are flowed downwardly through the shaft in the same direction as the travel of aggregate. The combustion gases pass through the crossover duct between the shafts and travel upwardly through the inactive shaft. After a period of time, the airflow is reversed and fuel is introduced into the other shaft. Thus, as used herein, the terminology "regenerative shaft kiln" shall be understood to refer to kilns of the type having at least two vertical shafts, wherein combustion air is flowed downwardly in shafts during their active phase, through a crossover between active and inactive shafts and upwardly through inactive shafts.
One challenge of regenerative or multiple shaft kilns is the initial or start-up phase of these kilns. Because these kilns are often configured to calcine several hundred tons of limestone per day and calcining requires a temperature of about 1750.degree. F., it can often take several days to obtain operating conditions within the kiln. Once the kiln is properly started it can typically run for long periods of time without significant adjustment. However, getting to that point requires considerable adjustment and activity on the part of the operator with considerable loss in equipment from damage and loss of quality product from down-time and waste from poor operating conditions. Difficulty in starting the kiln is typically a function of the fuel type and grade, with the more expensive fuels being easier to work with. For example, kilns using exclusively natural gas are typically easier to start up, but gas is considerably more expensive than coal. Also, European coal which is typically lower in volatile content than most coals found in the United States is typically less troublesome than U.S. coals, but much more expensive.
Another difficulty resides in control over the temperature within the kiln. For example, if the limestone is not subjected to sufficient temperature for sufficient time, it will not be turned into lime. Also, if the temperature is too high (above about 1950.degree. F.) the limestone will over burn and have lesser value. Still another relates to the introduction of fuel into the kiln. For example, it has been experienced that high cost lances used to introduce fuel into the kiln can be destroyed by overheating.
An attempt to overcome problems in kiln operation, particularly during start-up, has been to monitor the temperature at the cross-over, at the top of the kiln and along the height of the kiln using thermocouples embedded in the refractory material inside the kiln. This method has proved ineffective, as damage to components of the kiln, particularly fuel lances has been observed even when the measurements are within the desired range.
Accordingly it is an object of the present invention to provide an improved multiple shaft or regenerative kiln and a method for controlling such a kiln which avoids many of the disadvantages of conventional regenerative kilns.
An additional object of the invention is to provide a kiln of the character described and a method for operating such a kiln which facilitates operation of the kiln and avoids many of the problems associated with the use of particular fuels.
Another object of the present invention is to provide a kiln of the character described which enables monitoring of conditions adjacent fuel feed lances within the kiln.
A further object of the present invention is to provide a method for controlling conditions within the lime kiln in response to measured conditions within the kiln to avoid destruction of lances within the kiln.
Yet another object of the present invention is to provide an improved method for starting up a regenerative kiln.
Still another object of the present invention is to provide a kiln of the character described which is uncomplicated in configuration and economical.
A still further object is to provide a lance construction which is advantageous as compared to conventional lances.
Having regard to the foregoing and other objects, the present invention is directed to a regenerative shaft kiln. According to the invention, the kiln includes at least two vertical shafts. Each shaft of the kiln includes a pre-heating zone in communication with a source of aggregate for introducing aggregate into the kiln and a fuel introduction zone below the pre-heating zone.
A plurality of lances are provided within the fuel introduction zone in flow communication with a source of fuel for introducing fuel into the kiln. A combustion zone is provided below the fuel introduction zone, and a cooling zone is below the combustion zone. A crossover zone between the combustion zone and the cooling zone connects the shafts is in flow communication with the crossover zone of at least one other shaft.
A sensor is provided proximate each of a plurality of the lances, each of the sensors producing a first output signal having a magnitude and corresponding to a physical parameter of the kiln adjacent the sensor.
A significant aspect of the invention relates to the configuration and operation of lance systems which introduce fuel into the kiln via the kiln. This enables an operator to monitor the operating conditions of individual lances and to control the introduction of fuel into individual ones of the latices in response to the operating conditions.
For example, in a preferred embodiment, both the pressure within the lances and the temperature of the tip of each lance are monitored, with the operator instructed to watch for undesirable pressure and/or temperature increases which are indicative of undesirable plugging of the lance. In response to the operator becoming aware of high pressure and/or temperature readings for a given lance, the operator may take action to prevent damage to the expensive lances.
One response is to shut off the fuel to the indicated lance for the next active cycle (about 15 minutes) which action has been observed to alleviate the problem in many cases. Thus, the invention enables close observation over the operation and operating environment of the individual lances and enables the operator to take action and prevent equipment damage, which is expensive both in terms of equipment cost as well as in loss of production resulting from downtime and/or poor quality from inadequate process conditions.
In an alternative embodiment, a computer monitors the temperature and pressure measurements from each lance, displays those measurements and automatically shuts off or decreases fuel flow to the lance in response to temperature and pressure measurements that exceed predetermined criteria.